Since I haven't watched hardly any of the show, or read any of this thread... I'm just going to throw this out there and if it's been speculated on then I apologize...
I have this feeling that the island, or some force of nature, is controlling when people die - and I believe it has something to do with "completing a mission". Everyone on that island that survived the crash and the initial few minutes past it have/had a reason to get there, and once their reason was complete, they die. It might not be right away, but it's fairly close.
Some of them don't have really "HUGE" missions to complete (the pilot, Arzt), and others might even have more than one ( Desmond, Jack, etc) and no doubt there may be some things that happen next season that have not even been alluded to yet. And I have this feeling that there are some people that just are not supposed to die no matter what (for reasons we don't know yet, like Locke).
As far as the Others go, I am not sure whether or not they are "exempt" from the mission thing, but it's fairly easy to see that certain Others mean more to the island than other Others.